As a conventional engine, there is known a working gas circulation type engine as a so-called closed cycle engine which can circulate a working gas from a gas exhaust side to a gas suction side of a combustion chamber and can supply the working gas to the combustion chamber again. As the conventional working gas circulation type engine, for example, Patent Literature 1 discloses an internal combustion engine which combusts hydrogen as a fuel in a combustion chamber as well as uses an inert gas as a working gas. The internal combustion engine removes water vapor as a product contained in a gas discharged from the combustion chamber by a water vapor removing device, circulates the inert gas from which the water vapor has been removed to the combustion chamber again via a circulation path as well as returns the molecules of the inert gas dissolved in the removed water to a gas state by a deaeration device and supplies also the molecules in the gas state to the combustion chamber again.